The present invention relates generally to the modulation of optical signals and more specifically to a signal modulation system that samples and modulates a train of pulses.
Major applications where analog signals are transmitted over fiber include cellular (analog/digital/PCS), cable television, and electrical signals from antennae (such as satellite or radar signals). Unfortunately, analog signals are fragile. They are sensitive to dispersion, phase nonlinearities, scattering mechanisms and noise. Here we suggest a modulation format that mitigates these impairments. Consequently, analog signals can now travel longer distances with less degradation.
The current optical fiber universe appears to be dichotomy that is divided between a strict transmission of digital pulses, or in the alternative, a modulated carrier wave that forms an analog envelope. Samples of current technology are illustrated in the following U.S. Patents, the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 6,259,281 Jul. 10, 2001, Parallel analog sampling circuit and analog-to-digital converter system incorporating clock signal generator generating sub-sampling clock signals with fast and precisely-timed edges, Neff, Robert M.,
U.S. Pat. No. 6,263,016, Jul. 17, 2001, Methods for interfacing a subscriber link to digital networks, Bellenger, Donald Morgan,
U.S. Pat. No. 6,191,720, Feb. 20, 2001, Efficient two-stage digital-to-analog converter using sample-and-hold circuits, Zhang,
U.S. Pat. No. 4,660,192, Apr. 21, 1987, Simultaneous AM and FM transmitter and receiver, Pomatto, Sr., Robert P.
B. Wilson and Z. Ghassemlooy, IEE Proc J, vol 140, Dec. 1993, pp. 346-57.
Optical Fiber Telecommunications IIIA, Academic Press, 1997, edited by I. P. Kaminow and T. L. Koch.
W. I. Way's book Broadband Hybrid Fiber/Coax Access System Technologies, Academic Press, 1999.
Fiber impairments are described in G. P. Agrawal's book, Nonlinear Fiber Optics, Academic Press, 1995. An overview of how fiber may affect analog systems can be found in M. R. Phillips and T. E. Darcie's chapter in Optical Fiber Telecommunications IIIA, Academic Press, 1997, edited by I. P. Kaminow and T. L. Koch. W. I. Way's book, Broadband Hybrid Fiber/Coax Access System Technologies, Academic Press, 1999 discusses this as well as some analog applications. They are incorporated herein by reference.
The Pomatto reference illustrates classic analog signal amplitude modulation (AM). This reference imprints the information of a modulating signal onto a sinusoidal carrier signal to output a modulated signal. The modulated signal is the carrier signal with an amplitude envelope that represents the curve of the modulating signal. The receiver demodulates the modulated signal to retrieve the original modulating signal and the information contained therein.
Using pulses to convey analog (or arbitrary time varying) information is rare but it does exist. Prior implementations have carried information in the pulse's width, frequency, (temporal) position, or interval. Called pulse time modulation, this is reviewed by the Wilson reference. In particular, on p. 348, a diagram alludes to the use of pulse shape to convey information though I have not seen any actual work that does this. Our preferred embodiment uses different characteristics of the pulse (namely, amplitude or envelope) to carry information. This patent also notes that information can also be carried in the pulses' energy, phase, and polarization, too.